1. Field of the Invention
This invention relates to pile-driving apparatus for use in underwater pile-driving, and comprising a hammer unit in which the hammer or ram is moved downwards and upwards in a housing to deliver an impact to the pile through an opening in the bottom of the housing and in which the hammer is driven, at least in the upwards direction, by means of a pressurized driving liquid. The driving system may be single-acting in which the hammer is driven upwards by the driving liquid and then falls by gravity as a dropweight to deliver the impact force to the pile, or double-acting in which the hammer acts as a ram and is driven both upwards and then downwards by the pressurized driving liquid.
2. Description of the Prior Art
In the art of underwater pile-driving, the housing is closed and filled with air under suitable compression to act as a kind of diving bell so that the hammer can move in air and not in water. One such construction is described in British Pat. No. 1,388,690. The drive unit comprising motor-driven, positive-displacement type pumps and associated accumulators for pressurizing the driving liquid, usually oil, can be located above the water, e.g. on a ship as described in DOS No. 2,243,309. As the depth of pile-driving increases, pressure losses occur in the supply hoses and these pressure losses can theoretically be compensated for at greater depth; a depth of 300 meters is the present practical limit for pressurized liquid hoses, which are also very expensive.
Proposals have therefore been made to reduce the lengths of the supply hoses by locating the driving unit comprising the motor-driven pumps and associated accumulators on the hoisting frame of the hammer unit or other member which is so connected to the housing that it may move over a limited distance relative to the housing and, in some cases, is provided with buoyancy tanks (see U.S. Pat. No. 4,043,405).